Macromolecular crowding effects in the mechanical unfolding forces of proteins

Abstract

Macromolecules can occupy a large fraction of the volume of a cell and this crowded environment influences the behavior and properties of the proteins, such as mechanical unfolding forces, thermal stability and rates of folding and diffusion. Although much is already known about molecular crowding, it is not well understood how it affects a protein’s resistance to mechanical stress in a crowded environment and how the size of the crowders affect those changes.An atomic force microscope-based single molecule method was used to measure the effects of the crowding on the mechanical stability of a model protein, in this case I-27. As proteins tend to aggregate, single molecule methods provided a way to prevent aggregation because of the very low concentration of proteins in the solution under study. Dextran was used as the crowding agent with three different molecular weights 6kDa, 10 kDa and 40 kDa, with concentrations varying from zero to 300 grams per liter in a pH neutral buffer solution at room temperature. Results showed that the forces required to unfold biomolecules were increased when a high concentration of crowder molecules were added to the buffer solution and that the maximum force required to unfold a domain was when the crowder size was 10 kDa, which is comparable to the protein size. Unfolding rates obtained from Monte Carlo simulations showed that they were also affected in the presence of crowders. As a consequence, the energy barrier was also affected. These effects were most notable when the size of the crowder was 10 kDa, comparable to the size of the protein. On the other hand, distances to the transition state did not seem to change when crowders were added to the solution. The effect of Dextran on the energy barrier was modeled by using established theories such as Ogston’s and scaled particle theory, neither of which was completely convincing at describing the results. It can be hypothesized that the composition of Dextran plays a role in the deviation of the predicted behavior with respect to the experimental data.%%%%Ph.D., Physics – Drexel University, 2012